Method of making fiber composites

ABSTRACT

A method of making a fiber composite having high strength at 2000* to 2200* F. by slip casting a nickel alloy matrix slurry into an array of tungsten fibers. The slip is dried and sintered in dry hydrogen at 1500* F. for one hour. The resulting body is isostatically hot pressed at 20,000 p.s.i. and 1500* F. for one hour, followed by a second isostalic hot pressing at 2000* F. for 1 hour.

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Filed:

METHOD OF MAKHNG FHBER COMPOSIITES Inventors: Donald W. Petraseli, RockyRiver; Robert A. Signorelli, Strongsville; John W. Weeton, Rocky River;Gerald B. Beremannl, Avon, all of Ohio The United States of America asrepresented by the Administrator of the National Aeronautics and SpaceAdministration Feb. 27, 1970 Appl. No.: 15,222

Assignee:

Related ILLS. Application Data Division of Ser. No. 768,907, Oct. 18,1968.

75/DIG. 1 Int. Cl. ..B22i 3/14 Field of Search ..75/200 F, 208, 211, 226

Primary ExaminerBenjamin R. Padgett Assistant ExaminerB. H. HuntAttorney-N. T. Musial, G. E. Shook and G. T. McCoy [5 7] ABSTRACT Amethod of making a fiber composite having high strength at 2000 to 2200F. by slip casting a nickel alloy matrix slurry into an array oftungsten fibers. The slip is dried and sintered in dry hydrogen at 1500F. for one hour. The resulting body is isostatically hot pressed at20,000 p.s.i. and 1500 F. for one hour, followed by a second isostalichot pressing at 2000 F. for 1 hour.

8 Claims, No Drawings METHOD OF MAKING FIBER COMPOSITES RELATEDAPPLICATION This application is a division of copending application Ser.No. 768,907 which was filed Oct. 18, 1968.

STATEMENT OF GOVERNMENT OWNERSHIP The invention described herein wasmade by employees of the United States Government and may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

BACKGROUND OF THE INVENTION Advanced airbreathing engines need highstrength materials for use at temperatures above 1800 F. The tensile andstressrupture strengths of nickeland cobalt-base superalloys are lessthan those required by such engines at the high temperatures. Alloys ofthe refractory metals tungsten, molybdenum, columbium, and tantalum havethe required strength, but they are more prone to catastrophicoxidation. Ceramic or refractory compounds are subject to brittlefailure.

Metal fiber-reinforced superalloy composites have been proposed for useat high temperatures. Reactions between the fibers and the matrix areoften detrimental to the properties of the composite. The tensile andstress-rupture property values of such materials exhibit littleimprovement over the unreinforced superalloy materials.

Dry powder fabrication processes were also tested while the diffusionbonding of alternate layers of metal foil and fibers was likewiseconsidered. These procedures limit fiber content to about 50 volumepercent. Liquid phase methods have been utilized to achieve higher fibercontents. However, the liquid phase matrix materials can cause severefiber degradation.

SUMMARY OF THE INVENTION A high strength fiber composite material isprepared by slip casting a matrix powder-water slurry into an array offibers. The preferred matrix composition in weight percent is nickel 56,tungsten 25, chromium l5, titanium 2, and aluminum 2. The preferredfiber composition is tungsten with one weight percent thorium oxide. Theslip casting is dried and then consolidated by pressing and heating.Fiber contents are varied to achieve the desired reinforcement.

OBJECTS OF THE INVENTION It is, therefore, an object of the presentinvention to provide an improved method of making high strength fibercomposite material for use at elevated temperatures where weight savingsare desirable.

Another object of the invention is to provide a method of making a fibercomposite suitable for use where higher strength or greater strength todensity ratios are required than those exhibited by the superalloys.

A further object of the invention is to provide a method of making arefractory metal fiber reinforced superalloy material composite havingsuperior strength to density properties wherein the loss of fiberproperties within the matrix is limited.

These and other objects of the invention will be apparent from thespecification that follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the invention fiberreinforced composites are prepared by slip casting a superalloy matrixinto a bundle of fibers. Composites were prepared in this manner usingbundles of NF and 218CS wire. The composition of the NF wire wastungsten with 1 percent thoria. The 218CS wire was commercial tungsten.

The wire was received in the as-drawn, cleaned, and straightenedcondition. Wires having diameters of 0.008, 0.015, and 0.020 inch wereused.

The superalloy matrix compositions were selected to be compatible withthe wire material while taking into consideration forgeability andoxidation resistance at high temperatures. A large amount of refractorymetal was added to a nickel matrix to lower the reactivity with thereinforcing fibers by reducing the chemical potential differential fordiffusion. A high percentage of chromium was also added to the nickel toenhance oxidation resistance. The nominal compositions of the matrix inweight percent was nickel 56, tungsten 25, chromium l5, titanium 2, andaluminum 2.

Aluminum additions were made to the alloy to form a gamma prime phase(Ni Al), and titanium additions were made to these alloys to form an etaphase (Ni Ti). Both of these elements precipitation hardened the nickelalloy. The additions to the alloy were substituted for correspondingamounts of chromium. These additions tie up three atoms of nickel foreach atom added and further lower the reactivity of the matrix alloywith the fiber by lowering the nickel potential for diffusion.

The nickel alloy was vacuum cast and atomized into fine powder. Theparticle size of the metal powder was 325 to +500 mesh. A 2.5 percentsolution of a binder material comprising an ammonium salt of alginicacid in water was added to the metal powder to form a slip. The solutionwas then diluted with water so that the solid to liquid ratio wasreduced and the viscosity was lowered to the point where the slip waspourable.

The metal powder content of the slip was 89.9 percent by weight whilethe water content was 10.0 percent by weight. The binder content was 0.1percent by weight.

Continuous length refractory wire bundles were inserted into a nickeltube containing a wire screen at the bottom adjacent several layers offilter paper. The tube containing the wire bundles was connected to avacuum pump through a hose.

The nickel tube was placed on a vibrating table, and slip was pouredinto the wire bundle while the tube was vibrated. As the nickel-alloypowder settled to the bottom of the bundle excess liquid media wassiphoned off the top and more slip was added. This process was continueduntil the nickel-alloy powder completely infiltrated to the top of thewire bundle.

The vibrator was stopped, and a vacuum was applied to the tube to removeany additional liquid media left in the casting. The composite wasremoved from the tube and dried in air for approximately 24 hours at 140F.

The densification technique used on the composites consisted ofsintering the slip cast composite at l500 F. for one hour in dryhydrogen to drive off the binder and to reduce any nickel or chromiumoxide film that might be present on the surface of the powders. Aftersintering, the composites were sealed in nickel cans. Finaldensification was accomplished by isostatically hot pressing the billetsfirst at 1500 F. for 1 hour and then at 2000 F for 1 hour under heliumpressurized at 20,000 p.s.i.

Stress-rupture tests on single fibers were conducted in a stress-ruptureapparatus at 2000 and 2200 F. for periods up to 200 hours.Stress-rupture tests on vacuum-cast nickel-alloy specimens and oncomposite test specimens were conducted in conventional creep machinesusing a helium atmosphere to limit oxidation. These tests were likewiseconducted at 2000 and 2200 F. Tensile tests were conducted on the fibersand composites at 2000 F.

The stress to cause rupture at hours at 2000 and 2200 F. for the wirematerial and the nickel alloys is shown in Table 1. Also shown in thisTable is the stress to cause rupture at 100 hours divided by thematerial density (specific strength).

TABLE I.STRESSRUPTURE IN 100 HOURS Composites were produced using thewires and nickel alloy matrix shown in Table 1. These composites hadstress-rupture properties superior to conventional superalloys at usetemperatures of 2000 and 2200 F. Composite stress-rupture properties areshown in Table ll.

The 100-hour stress-rupture strength obtainable for compositescontaining 70 volume percent of either 0.015 inch diameter 2 l 8CS wireor 0.020 inch diameter NF wire at 2000 F. was 35,000 p.s.i. as comparedwith 11,500 p.s.i. for the best cast nickel alloys. At 2,200 F. the100-hour stress-rupture strength obtainable for the 70 volume percentfiber composite was 14,000 p.s.i. as compared with 4,000 p.s.i. for thecast nickel alloys. The 100-hour rupture strength for the composite at2,000 F. represents a use temperature advantage over cast nickel alloysof approximately 200 F.

TABLE II.-COMPOSITE STRESS-RUPIURE PROPERTIES Fiber Wire content,Stress, Li vol. Test Matl. Dia./in. p.s.i. hr percent temp.. F

The density of the composite material is much greater than that of thenickel alloy. The density of the material is important for certainapplications, such as in turbine blades where tensile stresses are aresult of centrifugal loading. Tungsten has a density about 2.3 timesthat of most nickel-base alloys, and a composite containing 70 volumepercent tungsten fibers has a density of approximately 1.9 times that ofnickel base alloys. The temperature advantage of the composite issomewhat reduced on a specific strength basis. However, on this basisthe 70 volume percent reinforced composite is more than 5 times asstrong for a 100-hour rupture life at 2000 F. than for the unreinforcednickel matrix alloy. Also, the 70 volume percent fiber reinforcedcomposite is approximately 60 percent better than the best case nickelalloys for rupture in 100 hours and 3 times as strong for rupture in1,000 hours. The 70 volume percent fiber reinforced composite is 2 timesas strong at 2,200 F. as the cast nickel alloys for 100-hour rupturelife and 2.5 times as strong for an expected l,00O-hour rupture life.

Tensile strength at 2,000 F. of composites made in accordance with theinvention are shown in Table III. Both 218CS and NF wires were used withthe nickel alloy matrix. The 218CS wire had a diameter of 0.015 and atensile strength of 111,000 p.s.i. at 2,000 F. The NF wire had adiameter of 0.020 inch and a tensile strength of 1 16,000 p.s.i. at2,000 F.

TABLE III.-COM1OSITE TENSILE STRENGTH AT 2,000 1'.

Fiber 1 Tensile content, Strength, vol Elongation, Wire Metl. p.s.i.percent percent NF 60,800 55.0 68,000 50.0 4.23 71,500 52.8 4. 57 77,00055.3 5.30 78,000 64.4 91, 500 07. 5 5. 49 70, 000 54. 0 4. 70 B3, 20060.3 4. 66,800 40. 2 4. 20

The nickel alloy matrix containing titanium and aluminum additions wasmore compatible with the 21 8G8 and NF fibers than nickel alloys whichdid not contain these additives. The reaction between the mutuallysoluble fiber and matrix material in the above composites was limited toapproximately 1.25 mils after exposure for hours at 2,000 F.

Wire diameter is also important in composites where reactions betweenthe fiber and matrix material occurs. The strength contribution of thereacted fiber in a composite can be related to the area fraction of thefiber that has been alloyed. In composites fabricated in accordance withthe present invention the strength contribution of the fiber decreasedas the area fraction of the alloyed portion of the fiber increased. Asthe fiber diameter increased, however, the unalloyed fiber strengthgenerally decreased. The method of the present invention takes intoaccount both factors, and the composite strength as a function of wiresize and compatibility of the matrix is predictable.

For short-time applications, small diameter fibers are more advantageousthan large diameter fibers. For long-time applications, large diameterfibers are superior.

What is claimed is:

l. A method of making a composite comprising the steps of assemblingtungsten fibers in a parallel array, each of said fibers having a lengthsubstantially equal to that of said composite,

pouring a slip comprising a nickel alloy matrix powder and water, saidnickel alloy matrix powder comprising tungsten, chromium, titanium andaluminum into said fiber array,

removing water from said slip to form a casting, said casting having afiber content greater than 50 volume percent,

sintering the slip casting in dry hydrogen at 1500 F. for 1 hour, and

canning a sintered slip casting,

isostatically hot pressing the canned sintered casting at about 1500 F.for about 1 hour, and

isostatically hot pressing the canned sintered casting at about 2,000 F.for about one hour at about 20,000 p.s.i.

2. A method of making a composite as claimed in claim 1 wherein thetungsten fibers contain about 1 percent thoria by weight.

3. A method of making a composite as claimed in claim 1 6. A method ofmaking a composite as claimed in claim 1 wherein the nickel alloy powderhas a particle size between --325 to +500 mesh.

7. A method of making a composite as claimed in claim l wherein the slipcontains a 2.5 percent solution comprising an ammonium salt of alginicacid in water.

8. A method of making a composite as claimed in claim l including thesteps of vacuum drying the slip casting, and

air drying said casting for about 24 hours at about F.

2. A method of making a composite as claimed in claim 1 wherein thetungsten fibers contain about 1 percent thoria by weight.
 3. A method ofmaking a composite as claimed in claim 1 wherein the fibers havediameters between 0.008 inch and 0.020 inch.
 4. A method of making acomposite as claimed in claim 1 including the step of slip castingnickel alloy powder into the fiber array, said alloy powder having anominal composition by weight of tungsten 25 percent, chromium 15percent, titanium 2 percent, aluminum 2 percent, and the restsubstantially nickel.
 5. A method of making a composite as claimed inclaim 1 wherein the nickel alloy powder comprises about 90 percent ofthe slip.
 6. A method of making a composite as claimed in claim 1wherein the nickel alloy powder has a particle size between -325 to +500mesh.
 7. A method of making a composite as claimed in claim 1 whereinthe slip contains a 2.5 percent solution comprising an ammonium salt ofalginic acid in water.
 8. A method of making a composite as claimed inclaim 1 including the steps of vacuum drying the slip casting, and airdrying said casting for about 24 hours at about 140* F.